Glow discharge apparatus



Oct. 2, 1962 w. A. LLOYD ETAL GLOW DISCHARGE APPARATUS Filed Jan. 8, 1960 INVENTORS William A. Lloyd Renn Zaphiropoulos United States Patent 3,056,902 GLOW DISQHARGE APPARATUS William A. Lloyd and Reun Zaphiropoulos, Los Altos, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 8, 1960, Ser. No. 1,377 17 Claims. (Cl. 315-169) This invention relates in general to glow discharge apparatus, and more particularly to cold cathode glow discharge devices, especially useful for pumping gases from within enclosed vessels to extremely low pressures and/ or measuring gas pressures to extremely low pressures.

Heretofore high capacity cold cathode glow discharge getter ion pumps have been made having an enlarged central chamber and a plurality of lesser chambers communicating therewith and extending outwardly therefrom. The pumping elements were arranged within the lesser outwardly directed chambers and included two elongated cathode plates having a cellular anode structure insulated from and disposed, in sandwich fashion, therebetween in spaced-apart relation. The cathode plates and sandwiched anode structure were assembled together via suitable insulators to form a pumping element canister sub-assembly. The complete high capacity pump had one of these pumping canisters sub-assemblies in each of the communicating lesser chambers. Permanent magnets were arranged around the outside of the pump envelope between the outwardly directed lesser communicating chambers for directing a magnetic field circumferally of the pump envelope and through the pumping canisters to enhance the glow discharge and thus the pumping action of the pump.

In the prior art pumping canister the two elongated rectangular cathode plates were held together in spaced apart relationship via machined blocks disposed at the opposite ends of the cathode plates. The cellular anode was supported at its ends from the machined blocks via two pairs of longitudinally directed frustroconical insulators. Spacer rods of cathode material were inserted coaxially of some of the cellular anode members midway along the length of and transversely directed of the elongated cathode plates to hold the proper spacing between the cathode plates. These spacer members were also fixedly connected at their ends to the paced apart cathode plates to prevent warping and distortion of the cathode plates as the cathode plates became hot in use.

A number of problems were encountered with the above-mentioned prior art pumping element canister. In particular, the canisters could not be made of indefinite length because the anode was supported with respect to the cathode plates only at the ends of the cellular anode. With only end support for long anodes, the center portion of the anode would warp due to thermal changes and also was free to deflect in an environment of shock and vibration. It was found that when the anode member deflected such that it was more closely spaced to one of the cathode plates than to the other mutually opposing cathode plate, the pumping speed of the canister was substantially impaired. Of course, if the anode deflected to the extent that it made physical contact, or near physical contact, with the cathode, it shorted out the pump causing severe damage.

The present invention provides means for mounting the cellular anodes with respect to the cathode plates such that the anode within the pumping element canisters can be made of indefinite length for operation in an environment of shock and vibration without impairing the performance thereof. In particular, novel insulator assemblies are provided which extend transverselyof the cathode plates. These novel insulator assemblies, in

3,056,902 Patented Oct. 2, 1962 addition to assuring proper spacing between the cathode plates, fixedly hold the anode at the proper spacing with respect to the cathode plates. Any desired number of these insulator assemblies may be provided lengthwise of the cathode plates whereby rigid pumping canisters of indefinite length may be fabricated.

The principal object of the present invention is to provide an improved cathode anode assembly for cold cathode glow discharge devices useful, for example, for measurement of extremely low gas pressures and/or for pumping gases to extremely low pressures.

One feature of the present invention is the provision of a novel cold cathode glow discharge cathode-anode assembly in which a cellular anode is supported from a cathode plate via the intermediary of an insulator body directed transversely of and interconnecting the anode and cathode whereby anode-cathode assemblies can be made of indefinite length.

Another feature of the present invention is the provision of a novel cold cathode configuration wherein an elongated cathode plate is provided with a transversely directed flange running longitudinally thereof to prevent warping and buckling of the cathode plate in use.

Another feature of the present invention is the provision of a novel high-voltage insulator assembly wherein a dielectric insulator body is connected at opposite ends to members at cathode potential and connected intermediate its length to an anode member for rigidly supporting therefrom said anode. The insulator body is provided with transversely directed corrugation therein for increasing the leakage path thereof. Sputter shield members are disposed coaxially of the insulator body between said insulator body and the anode member and are directed toward the center of the insulator body from the ends thereof. This insulator assembly is relatively simple, mechanically strong, and has a high leakage resistance.

Another feature of the present invention is the provision of a novel anode to cathode insulator assembly wherein a dielectric insulator body is fixedly secured at one end thereof to a member operating a cathode potential and extends transversely thereof. The insulator body is provided with transverse corrugations in the outside surface thereof to increase the leakage resistance thereof, and the insulator body is connected at the free end to the anode member for holding the anode in spaced relation from said cathode member. A cup-shaped sputter shield, operating a cathode potential, is coaxially disposed of the insulator body extending from the fixed end thereof toward the free end portion of the insulator body. This insulator assembly provides a rigid support for the anode from the cathode member and has an extremely high leakage resistance.

Another feature of the present invention is the provision of a cathode anode subassembly wherein there is provided means for changing the relative position of the anode and cathode members to obtain more even erosion of the cathode plates and thereby prolong the operating life of the cathode.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is an external isometric view of a high-capacity,

high-speed vacuum pump utilizing features of the pres-- ent invention,

22 in the direction of the arrows,

FIG. 3 is an isometric view, partly broken away, of

a cathode-anode canister sub-assembly embodying features of the present invention,

FIG. 4 is an enlarged detail view of a portion of the structure of FIG. 3 delineated by line 44 thereof,

FIG. is an isometric fragmentary view, partially broken away of an alternative cathode anode canister sub-assembly embodying features of the present invention,

FIG. 6a is a cross-sectional view of an alternative insulator assembly for that portion of the structure of FIG. 2 delineated by line 66 thereof, and

FIG. 6b is an alternative insulator assembly for the structure of FIG. 6a.

Referring now to FIG. 1, there is shown in isometric view the external portion of a high-speed, high-capacity getter ion vacuum pump. In particular, an enlarged central rectangular chamber 1 is provided with a plurality of outwardly directed lesser rectangular chambers 2 communicating therewith and disposed about the periphery of the enlarged central chamber 1. The outwardly directed lesser chambers 2 extend longituidnally of the pump body and are spaced apart, about the periphery of the pump, to accommodate a plurality of permanent magnets 3 disposed between adjacent lesser chambers 2 for providing a circumferally directed magnetic field to enhance the glow discharge and thus the speed of the pump. The pump is provided with an enlarged opening therein which is provided with a flanged adaptor 4 for connecting to structures it is desired to evacuate. Pumps of this general configuration can be built to have substantially unlimited pumping capacity. Similar pumps have been built having pumping speeds of 7000 liters per second, and being capable of pumping down to gas pressures below 1 l0 mm. of mercury.

Referring now to FIG. 2 there is shown an enlarged cross-sectional view of the lesser communicating chambers 2 containing therewithin the pumping elements. In particular, the outwardly directed lesser chamber 2 contains two mutually opposed spaced apart rectangular cathode plates 5 disposed along opposite broad sides of chamber 2. The cathode plates 5 may be made of any suitable gettering material, such as for example, titanium, magnesium, zirconium, or chromium. The cathode plates 5 are made to have a suitable thickness, as, for example, 0.090 of an inch thick and are provided on their innermost sides with transversely outwardly directed flange portions 6. The flanges 6 serve the threefold purpose of strengthening the cathode plates 5 against warping and buckling, as a stop for determining the maximum extent of penetration into the chambers 2, and as a tab which may be gripped by suitable means for holding the cathode elements within the lesser chamber 2 to keep them from falling out when the pump is moved or shipped.

A cellular anode assembly 7 is disposed between the two cathode plates 5 in spaced apart relation therefrom. The individual cells making up the cellular anode 7 preferably have a depth greater than their characteristic transverse dimension and their axial center line is preferably substantially in alignment with the magnetic field H, which threads through the lesser chamber 2 substantially transversely to the cathode plates 5, and which is produced by the permanent magnets 3. The preferred characteristic transverse dimension for the individual cells is a function of the magnetic field intensity H and the applied voltage between anode 7 and cathode members 5, but for a magnetic field intensity of 2000 gauss it has been found that a characteristic transverse dimension of approximately a half inch provides a good pumping speed. The cellular anode 7 is preferably made of the same material utilized for making the cathode plates 5 in order to minimize flaking of condensed sputtered cathode material produced by the cold cathode glow discharge in use. The anode 7 is preferably provided with a metallic tab 8 for making electrical connection to the high voltage power supply, not shown, for supplying the operating potentials as of MOO-10,000 volts applied between cathode plates 5 and anode 7.

Insulator assemblies 9 serve the threefold purpose of providing the proper spacing between mutually opposing cathode plates 5 to support the anode 7 in spaced apart relation from the cathode plates, and to stand off the high voltage applied between anode 7 and cathode plates 5. With the insulator assemblies 9 in place, the two cathode plates 5 together with centrally disposed anode 7 form a sub-assembly which has substantial mechanical strength and which may be readily removed, worked on and/ or replaced, as desired.

The insulator assemblies 9 include an elongated centrally bored dielectric insulator body 11 as of, for example, alumina ceramic. The insulator body 11 is provided with transverse corrugations 12 in the outside surface thereof to increase the current leakage path thereacross and thereby increase the leakage resistance thereof.

Two cup-shaped centrally bored insulator shield members 13 are disposed coaxially of and spaced apart from the sides of the insulator body 11. The shield members 13 extend over the insulator body 11 from the end portions thereof toward the central portion of the insulator body 11. The insulator shields 13 are preferably made of the same material as the cathode plates 5 to prevent flaking of condensed sputtered cathode material thereon.

A connecting rod 14 extends through the central bore of the insulator body 11 and through central openings in the insulator shields 13 and thence through suitable openings provided in the cathode plates 5. Cathode plates 5 preferably have an enlarged milled out portion on the back side thereof around the openings therein through which the connecting rod 14 protrudes. This enlarged milled out cathode portion around the protruding ends of the connecting rods 14 permits access to the protruding ends of the connecting rod 14 for receiving thereon suitable gripping retaining rings 15 which serve to fixedly hold together the entire anode-cathode sub-assembly.

The cellular anode 7 is connected to the insulator bodies 11 via centrally apertured anode headers 16 fixedly secured to the cellular anode, as by, for example spot welding. The transverse anode headers 16 slide over the insulator bodies 11 and are fixedly secured thereto by two retaining rings 17 carried from the insulator bodies 11 and capturing the anode headers 16 therebetween. The retaining rings 17 may be either of the gripping variety for gripping the insulator body 11, or of the non-gripping type which ride within a central transverse peripheral recess in the insulator body 11.

Referring now to FIG. 3, there is shown in isometric view an anode-cathode canister sub-assembly, as shown in FIG. 2. A convenient number of insulator assemblies 9 are disposed transversely of the cathode plates 5 and extend coaxially through the anode cells to provide a relatively rigid and mechanically strong anode-cathode canister. The canister is inserted within the lesser chambers 2, see FIG. 2, and is clamped therein by a plurality of clamping plates 21 carried from studs 22 and the clamping plates 21 are tightened down against the cathode plate fianges 6 as by, for example, nuts 23. In another embodiment of the present invention, not shown, the clamping plates 21 and nuts 23 may be replaced by gripping retaining rings fixedly secured over the studs 22 and extending transversely therefrom a sufiioent distance to clamp the cathode flanges 6 against the interior surfaces of the central chamber 1.

Referring now to FIG. 5, there is shown another embodiment of the present invention. In particular, the insulator assemblies 9 are shown disposed at the ends of the cellular anode 7 and are connected thereto by a transverse header 25. The insulator assemblies 9 are disposed at the ends of the cellular anode 7 to allow maximum utilization of the available magnetic field, which is generally expensive to produce.

In operation, a suitably high positive voltage is supplied to the cellular anode 7 as, for example, four thousand volts with respect to the cathode plates 5 which are typically operated at ground potential. In the presence of the magnetic field as of, for example, 2000 gauss threaded coaxially of each of the individual cellular anodes, a glow discharge is initiated which produces bombardment of the reactive cathode plates 5 by high-speed positive ions. Bombardment of the cathode plates 5 by the high-speed positive ion produces sputtering of the cathode material.

This sputtered cathode material is collected upon the rather large area of the cellular anode and there serves to getter active gases coming in contact therewith. The individual cells, which in the aggregate, make up the cellular anode 7, can be made to havean intrinsic pumping speed of approximately 0.8 liter per second. By suitable design of the pump with regard to impedance to gas flow, the intrinsic speed of the individual cells may be advantageously utilized. Pumping speeds for the entire apparatus can be realized which are generally proportional to the number of cells used.

After a substantial period of operation, of the abovedescribed cold cathode glow discharge vacuum pump, it will be found that the cathode plates 5 have been eroded away at a plurality of points on the mutually opposed faces thereof. The points of cathode erosion are found to be in substantial alignment with the center line of each of the individual cells making up the cellular anode 7. It has been found that the individual cells of the anode 7 tend to focus the ion bombardment of the cathode plates 5 along the center lines of the cells.

Eventually continued ion bombardment will erode through the cathode plates 5 and thereby deleteriously affect the pumping speed of the pump and/or damage the pump envelope. However, because of the rather sharp focusing of the bombardment ion beam, a large proportion of the cathode plates 5 remains unused. Accordingly, the cathode plates have been milled out on the backside thereof and a plurality of assembly holes placed therein, see FIG. 4, said holes being approximately one-half cell diameter apart.

By reassembling the cathode plates using a different set of assembly holes, the points of ion bombardment on the cathode plates can be changed to a new location as of, for example, spaced away by one-half a cell diameter for each of the three other assembly hole positions. In this manner the useful life of the cathode plates 5 can be prolonged to four times the life of a cathode plate having only one set of assembly holes, whereby a more economical use of the cathode material is obtained.

Referring now to FIG. 6a, there is shown an alternative insulator assembly 26 of the present invention. In particular, this embodiment is similar to the insulator assembly shown in FIG. 2 with the exception that the insulator body 11 of FIG. 2 has been separated into two mating parts 27 and 28. The upper insulator part 27 is provided with a cylindrical re-entrant portion 29 for mating, in abutting relationship, with a cylindrical protruding portion 31 of the lower insulator body 28. The upper and lower insulator portions 27 and 28 respectively, when assembled, serve to provide a central peripheral recess therebetween for gripping and holding the transverse anode header 16 therebetween. Insulator assembly 26 operates substantially identically to previously described insulator assembly 9, with the exception that the two centrally disposed retaining rings, which are utilized in the insulator assembly 9, have been dispensed with and replaced by the recess formed between upper and lower insulator members 27 and 28 respectively.

Referring now to FIG. 6b, there is shown an alternative insulator assembly 32 of the present invention. In particular, the insulator assembly 32 includes an elongated transversely corrugated dielectric insulator body 33 as of, for example, alumina ceramic. A stud 34 is formed in one end of the ceramic and extends through suitable openings in a cup-shaped insulator sputter shield 35 and cathode plate 5. The insulator body 33, and shield 35, are fixedly secured to and transversely directed from the cathode plate 5 via a gripping retaining ring 36 fixed over the protruding portion of the stud 34 at the back side of the cathode plate 5. The other free end of the insulator body 33 is formed with an outwardly directed flange portion 37 which catches on the inside periphery of the central aperture in an anode header 38, which in turn is fixedly secured to the cellular anode 7, as by, for example, spot welding.

The anode 7 is captured on the insulator body 33 via a gripping retaining ring 39 fixedly carried at the free end of the insulator body 33 and sandwiches the anode header 38 between the gripping ring 39 and the insulator flange 37. The preformed dimensions of the insulator body 33 determine the proper spacing of the anode 7 from the cathode plate 5. The other cathode plate 41 may be properly spaced from the anode 7 via other indexing devices. This insulator has the advantage of providing an extremely high leakage resistance due to the long insulator body length. In addition, this particular insulator assembly allows the anode 7 to be rigidly supported from only one cathode plate 5, which is desirable in certain pump configurations.

Although the insulator assembly embodiments of the present invention have been described as applied in a cold cathode getter ion vacuum pump, they may be utilized with equal facility and advantage in similar discharge devices, such as, for example, glow discharge ion gauges.

Since many changes could be made in the above construction and many apparently widely difierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A magnetically confined glow discharge apparatus including, means forming a gas-tight envelope defining a relatively large central chamber and a plurality of lesser outwardly directed chambers communicating with said central chamber, said individual lesser chambers having two mutually opposed relatively broad side wall portions and a relatively narrow end wall portion, means forming a cathode having spaced apart mutually opposed portions disposed parallel to the broad side walls of said lesser chambers, means forming an anode disposed in a plane substantially parallel to said broad side walls of said lesser chambers between and in spaced apart relation from said opposed cathode portions, means forming an insulator assembly supporting said anode means from at least one of said mutually spaced apart cathode portions, said insulator assembly being disposed transversely of the plane of both said supporting cathode portion and said anode means to rigidly support said anode with respect to said cathode means and to assure the proper transverse spacing between said anode means and said cathode means, means for producing and directing a magnetic field transversely of the plane of said anode means for magnetically confining and trapping electrons produced in the glow discharge between said spaced cathode portions, and means for applying a relatively high positive potential to said anode with respect to said cathode portion to produce a glow discharge at reduced gas pressures within said envelope in the space between said spaced cathode portions.

2. In a glow discharge apparatus, means forming a cathode having mutually opposed spaced-apart cathode surface portions, the width of said mutually opposed cathode surface portions being greater than the distance therebetween, means forming an anode disposed between and spaced apart from said opposed cathode portions, means forming an insulator fixedly secured at one end thereof to one of said opposed cathode surface portions and extending outwardly therefrom toward said other opposed cathode portion, means for supporting said anode means from said insulator means to provide a rigid support for said anode means with respect to said cathode means and to provide the proper spacing between said anode and said cathode means, and means for supplying a relatively high positive potential to said anode means with respect to said cathode means for establishment of a glow discharge in the spaces between said spaced apart cathode portions at relatively low gas pressure.

3. Apparatus according to claim 2 wherein said insulator means includes, a dielectric member provided with transversely direct corrugations in the outside surface thereof to increase the current leakage path thereof.

4. Apparatus according to claim 2 including, means forming a shield closely spaced coaxially adjacent to and externally of said dielectric member for shielding said insulator from sputtered cathode material which would otherwise condense on said dielectric member and reduce the leakage resistance thereof.

5. The apparatus according to claim 4 wherein said insulator means has one free end portion, and said anode support means is connected to said insulator means substantially at said free end portion thereof.

. 6. Apparatus according to claim 4 wherein said insulator means physically interconnects said mutually spaced apart cathode portions, and said anode support means is connected to said insulator means centrally of said insulator means.

7. Apparatus according to claim 6 wherein said anode support means is connected to said insulator means via a pair of retaining rings carried from said insulator means and disposed on opposite side of said anode support means for capturing said anode support means therebetween on said insulator means.

8. Apparatus according to to claim 6 wherein said anode support means is connected to said insulator means by being captured in a peripheral recess formed by spaced apart portions of separate upper and lower insulator portions of said insulator means pressed together in abutting mating relationship.

9. Apparatus according to claim 6 wherein said insulator shield means includes, a pair of tubular members coaxially disposed externally of and adjacent to said insulator means, and said tubular members extending lengthwise of said insulator means from the ends thereof toward the central portion of said insulator means.

10. An insulator assembly for glow discharge apparatus including, an elongated dielectric insulator body, said body having transversely directed corrugations in the outside surface thereof to increase the leakage resistance thereof, "a pair of tubular insulator shields coaxially disposed externally of and adjacent to said insulator body, said insulator shield extending longitudinally of said insulator body from the ends thereof toward the central portion of said insulator body, a pair of retaining rings centrally disposed of and carried from said insulator body for capturing therebetween a first metallic member of a certain high potential with respect to the potential at the ends of said elongated insulator body.

11. The apparatus according to claim 10 wherein said dielectric insulator body has a longitudinal bore therethrough, a rod disposed within said longitudinal bore, second metallic members disposed at the ends of said insulator body and apertured with the apertures in alignment with the bore in said insulator body, and the ends of said rod protruding through the apertures in said second members, and members fixed over the ends of said rod for holding said second members to said insulator body.

12. An anode-cathode sub-assembly for glow discharge apparatus including, a pair of spaced apart cathode plates, an anode member disposed between said cathode plates and spaced apart therefrom, a dielectric insulator body disposed between said cathode plates for holding said cathode plates apart in spaced relationship, a pair of tubular sputter shield members disposed over the ends of said insulator body, said sputter shield members extending from the ends of said insulator coaXially thereof toward the central portion thereof for shielding said insulator member from sputtered cathode material, means for securing said cathode plates to said insulator body, and means centrally disposed of said insulator body for physically connecting said anode to said insulator body whereby an anode-cathode sub-assembly is produced which is mechanically strong.

13. The apparatus according to claim 12, wherein said insulator body includes a longitudinal bore therethrough, and wherein said means for securing said cathode plates to said insulator body includes a rod disposed within the bore in said insulator body, the ends of said rod protruding from the ends of said insulator body and through aligned apertures in said cathode plates, and means fixedly secured over the ends of said rod for capturing therebetween said cathode plates, said insulator body, and said sputter shield members.

14. The apparatus according to claim 12 wherein said centrally disposed connecting means includes a pair of retaining rings secured to said insulator body and capturing therebetween said anode member.

15. The apparatus according to claim 12 wherein at best one of said cathode plates is provided with a transversely directed flange extending along one side thereof for strengthening said cathode plates against thermally produced distortions in use.

16. The apparatus according to claim 12 wherein said anode includes a plurality of closely grouped openings therethrough for the establishment of a glow discharge therethrough in use, and said means for securing said cathode plates to said insulator body includes means for selectively adjusting the relative position of at least one of said cathode plates with respect to said anode whereby the relative position of the glow discharge with respect to the cathode plates may be altered as desired to produce a more uniform erosion of the cathode plates by the glow discharge thereby extending the life of said cathode plates.

17. The apparatus according to claim 16 wherein said means for selectively adjusting the relative position of said cathode plate with respect to said anode includes a plurality of assembly holes in said cathode plate one of said assembly holes being utilized selectively as desired for securing said cathode plate to said insulator body.

References Cited in the file of this patent UNITED STATES PATENTS 

